Adds Prettier formatting and PHP support

Adds Prettier for consistent code formatting across the project.
Includes PHP plugin and configures formatting for PHP files.
Adds format and format:check scripts to package.json for ease of use.

package.json

@@ -11,9 +11,13 @@
     "build": "npx @tailwindcss/cli -i ./src/css/input.css -o ./assets/css/styles.css --content './site/**/*.php'",
     "serve": "php -S localhost:8000 vendor/getkirby/cms/router.php",
     "sync": "npx browser-sync start --proxy \"localhost:8000\" --files \"site/**/*.php,assets/**/*.css,assets/**/*.js,content/**/*.txt\" --no-open",
-    "dev": "concurrently \"npm run serve\" \"npm run sync\""
+    "dev": "concurrently \"npm run serve\" \"npm run sync\"",
+    "format": "prettier --write \"site/**/*.php\" \"content/**/*.txt\"",
+    "format:check": "prettier --check \"site/**/*.php\" \"content/**/*.txt\""
   },
   "devDependencies": {
-    "concurrently": "^9.2.0"
+    "@prettier/plugin-php": "^0.24.0",
+    "concurrently": "^9.2.0",
+    "prettier": "^3.6.2"
   }
 }